Voltage controlled oscillators (VCOs) are devices that generate an oscillating signal at a desired frequency. Unfortunately, these devices typically produce a certain amount of phase noise or jitter in the output oscillator signal, which is undesirable in many applications. One approach to solving this problem is to combine the outputs of two or more synchronously tuned VCOs to reduce the phase noise. With this technique, however, when the combined outputs are fed to a buffer circuit to drive a transmission line, the buffer circuit can re-introduce additional phase noise and reduce any gains achieved through the use of multiple VCOs.
Another problem with VCOs is that they can generally only be tuned over a relatively narrow frequency range. Outside of this range, the phase noise performance can become unacceptably large. Thus, many VCO devices employ multiple VCOs, each designed to cover a portion of the desired tuning range, and the VCO outputs are fed through a multiplexer network to select the output of one the VCOs which is associated with the desired frequency. Here again however, the multiplexers typically introduce additional phase noise which can at least partially defeat the purpose of using multiple VCOs, with this approach.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art.